The present invention relates to an intensity modulated optical signal generator which is capable of producing a linearized broadband optical signal, as required for frequency division multiplexed (FDM) amplitude modulated (AM) video.
With the advent of optical fiber technology wherein single mode optical fibers are now commonly available for transmitting information, a need exists for high bandwidth optical signal generators. Numerous high bandwidth optical signal generators have been proposed, however, each has its own drawbacks. Specifically, distributed feedback lasers are available for generating high bandwidth optical signals, but a disadvantage of such lasers is that the resulting high bandwidth signal has an undue amount of distortion and an undesirably low optical power output, especially when used for generating AM modulated signals.
It has also been proposed to generate a high bandwidth optical signal by utilizing a constant output laser and modulating an output thereof utilizing an external modulator, preferably a Mach-Zehnder interferometer. This technique also results in substantial signal output nonlinearities which are received as distortion. Johnson et al., U.S. application Serial Nos. 07/343,039; 07/412,656; and "Reduction of Intermodulation Distortion in Interferomic Optical Modulators" Optic Letters, Vol. 13, No. 10, October 1988, the disclosures of which are all incorporated herein by reference, disclose a method for substantially reducing nonlinearities attributable to third order products by adjusting relative amounts of the transverse electric (TE) and transverse magnetic (TM) power of an optical signal propagating through the external modulator to cancel the cubic dependence of the output optical power on drive voltage. More specifically, Johnson et al. propose to control the ratio of the power in the TE and TM modes by adjusting the polarization angle of polarized light at the optical input to the modulator such that the third order product of the modulated TE mode substantially cancels the third order product of the modulated TM mode. A preferred implementation of this technique is for use with a Mach-Zehnder interferometer.
Polarization control of a single external modulator to reduce third order products has two significant drawbacks. First, a frequency response of external modulators, in particular Mach-Zehnder modulators fabricated in lithium niobate (L1NbO3), is significantly different in the TE and TM modes. This effect ultimately limits the performance of the linearized modulator for broadband applications and is most significant below 200 MHz. Second, minimum second order distortion is only achieved when inflection points of the TE and TM transfer curves are precisely coincident and when the DC bias voltage is aligned to these inflection points. Unless the input polarization is precisely controlled electronically, the bias voltage must be used to achieve third order cancellation. However, since the bias control is optimally set to minimize second order products, using it to minimize third order products results in a direct tradeoff between second and third order distortion. Both cannot be optimized simultaneously with an external modulator unless the modulator is specially fabricated to very rigid specifications. In applications that demand very low distortion the single modulator implementation is most useful only for sub-optic applications where the second order distortion components fall out of band and can be filtered out electronically in a receiver.